TRANSACTIONS OF THE SECTIONS. 89 
B. decussatus, and B. striatus, Leda attenuata, Posidonia Becheri, Strophomena creni- 
stria, Chonetes Hardrensis, and Lingula sguamiformis; but, as we ascend upward, stray 
fragments of plants are mingled with these organisms, and on an endogenous leaf we 
find the marine Annelid Spirorbis carbonarius; further upward, the marine organiems 
decrease, and the plants increase ; and in the upper part of the deposit the marine 
organisms disappear, and numerous fragments of Stigmaria jicoides, Bechera, ferns 
of the genus Sphenopteris, and endogenous leaves are spread out between the 
layers of the shale. The Mountain Limestone of Northumberland is rolonged in a, 
narrow tongue for a few miles along the Berwickshire coast, and t ere, too, is a 
shale-bed_showing a similar assemblage of organisms; the lower layers contain 
Chonetes Hardrensis and Nucula gibbosa, but the upper layers are filled with Sphe- 
nopteris, reed-like stems accompanied with Spirorbis, Holoptychius Hibberti, and 
other fish-remains, and Estheria striata, var. Tateiana (Jones). Such sections 
indicate a change of conditions taking place while the beds were in course of de- 
* “\position : at first the conditions were undoubtedly marine, but they became estua- 
ring from some unknown cause—probably from a gradual alteration in level and 
‘in influx. of fresh water, and passed eventually into entirely freshwater conditions. 
On the Origin of the Jointed Prismatic Structure in Basalts and other Igneous 
_ Rocks. By Professor J. Toomson, M.A. 
The author gaye reasons against the prevailing views of the origin of the jointed 
prismatic structure of basalt and other igneous rocks, which are founded ‘on the 
supposition of a spheroidal concretionary tendency in the material during consoli- 
dation; or on this, combined with a tendency to split into prisms by shrinkage, 
such as is met with in the drying of clay or starch. 1st. No reason has been as- 
signed, and he believes none is conceivable, why the supposed centres of concre- 
tionary action should be arranged in straight or nearly straight rows, like beads on a 
vast number of parallel strings. 2nd. Even if the centres were so arranged, still we 
ought to expect, under the supposition in question, that the centre of a spheroid of 
one column would often be in front of the division between two spheroids of an 
adjoining column, and that thus the sides of the columns would be serrated with 
indentations and protuberances at the cross joints instead of being smooth as they 
actually are. He supposed that the columns of basalt have been formed by split- 
ting through shrinkage of a very homogeneous mass in cooling; and that the cross 
joints are fractures which have commenced in the centre of the column, and ad- 
vanced to the outside as a circle increasing in diameter. This mode of fracture he 
thought is evidenced by various markings and other indications on the stones. T hey 
usually show a remarkably symmetrical appearance round the outer part of their 
cross joint faces; presenting an appearance like a complete circular conchoidal frac- 
ture, often with rays from the centre, such as are seen in the ordinary conchoidal 
fracture. In order to produce the cross fractures commencing in the centre, he 
supposed that a longitudinal tensile stress must have existed in the columns pre- 
viously to the cracking of the cross joints. Ife would not venture to explain the 
origin of this tensile stress; but suggested that, perhaps, after the column was 
formed, chemical action, caused by infiltration of water, might cause a slight ex- 
pansion of the outside of the column, and so introduce the internal tensile stress. 
On “the Wash,” a remarkable Denudation through a Portion of the,Coal- 
Sidd of Durham. By Nicxotas Woon, F.G.S., President of the Institute 
of Mining Engineers of the North of England ; and Evwarp F. Bovyp. 
In the introductory part of this paper the authors observed that in all parts of 
the earth numerous proofs are exhibited of the abrading action of water, by which 
not only the valleys and softer strata, but the more consolidated strata also and 
the highest elevations of the land have been abraded; and, by this agency, vast 
accumulations of sand, gravel, clay, and rounded boulders have been formed and dis- 
tributed, by means of floods, over different parts of the earth’s surface. The authors 
do not jwish, in this communication, to determine the precise mode by which 
these accumulations have been formed, but only to point out the universality of 
